1. Field of the Invention
This invention relates to nanostructured austenitic steel and to a method of making nanostructured austenitic steel.
2. Background of the invention
Fine-grained materials have been investigated for their high strength, high toughness and other extraordinary properties. Two basic methods for refining the grain size of bulk steel to obtain ultrafine or submicron structures are severe plastic deformation and thermomechanical processing. Severe plastic deformation can be used to obtain submicron-grained microstructure (less than 1 mm) in steel. Examples can be found U.S. Pat. No. 4,466,862, German patent. 33 12257 and French patent 2524493, which disclose a hot-rolled low carbon steel having an ultrafine grain size of 3 μm or less. Japanese patent 2000073 1 52 discloses a method to produce ultrafine grained materials by accumulative roll-bonding, in which the microstructure with ultrafine grains of 0.5 μm in low carbon steel is obtained at an accumulative strain of about 5.6.
Thermomechanical processing can be used to obtain ultrafine ferrite (UFF) grains (about 1 mm) in steels through hot rolling in the intercritical region, warm rolling in the ferrite region, or strip rolling under the condition of strain-induced transformation from austenite to ferrite. Samuel et al [I.S.I.J. Int.,1990,30,216] reported that torsion testing of Niobium microalloyed steels produced austenite and ferrite grain sizes of 5 μm and 3.7 μm, where, dynamic transformation of austenite and/or dynamic recrystallization of ferrite takes place due to hot working. U.S. Pat. No. 6,027,587 discloses a process to produce an ultrafine grain by strain-induced transformation to ultrafine microstructure in ferrite steel. Ma et al [Scripta Materialia, 2005, 52, 1311] disclosed a repetitive thermomechanical process to produce austenite stainless steel with grain size down to 200 nm, in which the process was characterized by cold deformation and annealing cycles, combined with the addition of 8% Mn to enhanced austenite nucleation.
One disadvantage of ultrafine grained materials is that they have low elongation characteristic due to little work hardening ability. Therefore the manufacture of ultrafine grained materials needs large amount of plastic-working energy, special procedures, and complicated manufacturing techniques, which lead to high cost and limitations in material dimensions, mass production, and sample preparation, amongst other things.